Isoflavones, phytoestrogens found primarily in soy foods, are thought to protect against breast cancer, but epidemiologic evidence for such an effect is inconsistent. Inter-individual variability in isoflavone metabolism might explain some of the inconsistent results. Between 20 and 60% of individuals have intestinal bacteria capable of metabolizing daidzein, one of the primary soy isoflavones, to equol. Equol has been observed in vitro to demonstrate stronger estrogenic and antioxidant properties than either its precursor or the other primary soy isoflavone, genistein. Because of its strong association with breast cancer, breast density is a useful marker for breast cancer risk. The proposed project will examine equol producing status in relation to breast density and evaluate whether the association between isoflavone intake and breast density differs by equol producing status in a unique cohort of US Chinese women. A secondary, more exploratory aim is to examine another daidzein metabolite, O-desmethylangolensin (O-DMA), in relation to breast density. The work will include 280 recently immigrated US Chinese women participating in a separately funded, longitudinal study on diet and breast density. Study participants are pre-/peri-menopausal, of mammography screening age, with US residence <=20 years. Data being collected for that study include questionnaires and dietary interviews, blood samples, and a screening mammogram examined for breast density. The proposed work will add collection of overnight urine samples, laboratory analyses to define equol and O-DMA producing status, and statistical analyses to examine cross-sectional associations of equol and O-DMA producing status and dietary isoflavones with breast density measures already being collected as part of the diet-breast density study.